1. Field of the Invention
The present invention relates to an apparatus for measuring a displacement, and more particularly, to an apparatus for measuring a two-dimensional displacement.
2. Description of Related Art
The conventional two-dimensional displacement measuring module consists of two linear displacement measuring modules disposed orthogonally with respect to each other, the linear displacement measuring module being a common apparatus such as an optical linear scale encoder, a magnetic scale encoder, a linear variable differential transformer (LVDT) or a laser interferometer. However, the aforesaid two-dimensional displacement measuring module is suitable for the low-precision systems only, and has difficulty in meeting the requirement for being used in the high-precision systems. For example, there are problems in orthogonality precision between the two axes and orthogonality between a beam splitter/mirror and the optical path of an interferometer. Because of the orthogonal arrangement, highly-skilled technicians are always necessary for conforming to the requirement for a high-precision system. Therefore, costs for education and training of staff as well as risk of personnel outflow are increased.
Although there are currently several methods for improving the two-dimensional displacement measuring apparatus, a number of defects in efficiency exist. For example, U.S. Pat. No. 5,204,524 discloses a two-dimensional displacement measuring apparatus, in which both resolution and precision are limited by geometrical optic lens, to be further used at the high precision level. U.S. Pat. No. 5,424,833 also discloses a two-dimensional displacement measuring apparatus in which beams are diffracted three times with the undesired result that the luminance is poor, thus demanding finely manufactured components and assembly of the components to be involved in the system. U.S. Pat. No. 5,530,543 discloses a two-dimensional displacement measuring apparatus in which self-compensating function for errors are absent so that outputting signals are likely to be unstable in practice.
In addition, U.S. Pat. No. 6,744,520, to the inventors of the present invention, entitled Method for Measuring Two-dimensional Displacement by Conjugate Optical Paths, is provided to solve the defects of the aforesaid two-dimensional displacement measuring apparatuses. The method for measuring two-dimensional displacement by conjugate optical paths particularly concerns a method for measuring two-dimensional displacement, being tolerant of low fitting precision between optic lens and high alignment error between the optic lens and diffraction gratings. However, this two-dimensional diffraction means causes a plurality of diffracted beams in which a second-order diffracted beam (i.e., (2.0)-order diffracted beam) generated as a result of the first diffraction of the light to be very close to a second diffracted beam. If a laser light source having a longer coherent length is used, a combined wave signal consisting of two periodic wave interference signals will be formed, as shown in FIG. 1. It is therefore difficult for the second diffracted beam to be taken out individually.
Hence, there is a dire need to provide a two-dimensional displacement measuring apparatus where periodic wave interference signals formed relative to a displacement of the two-dimensional diffraction means are taken out easily so as to avoid an interference from occurring between different-order diffracted beams.